Environmental conditions combine with intrinsic neuronal factors to assemble and shape connections within the central nervous system. Auditory experience has been shown to be critically important for normal development of the central auditory pathway. Initial work performed as part of this CIDA indicates that significant structural changes occur in higher auditory neurons in the deaf dalmatian, a model of congenital deafness. The proposed project will extend this research theme by investigating auditory nerve function and synaptic morphometry in other animal models of congenital deafness. Auditory deafferentation and congenital hearing impairment will be used to explore the hypothesis that presynaptic activity modulates the form of synaptic contact, potentially regulating post-synaptic elements. The rationale for this focus is based on observations that 1) synaptic elements of primary auditory fibers with low versus high spontaneous discharge rates exhibit differences in their form, and 2) synaptic structure is subject to rapid changes in response to changes in the auditory environment. The potential role of certain structural proteins shown to modulate synaptic architecture in the visual system will be investigated in the auditory system using immunohistochemical methods. This application for continued CIDA support is based upon the proposed neuroanatomic studies and further training in light- and electron-microscopic techniques. This training will be used to continue the study of congenital sensorineural hearing impairment and factors that narrow and expand its habilitative potential.